Conventionally, a resin gear fabricated by an injection molding has been devised to control a flow of a molten resin material inside a cavity such that a molten resin is simultaneously filled into a cavity portion forming a tooth portion, thus improving a shape accuracy such as a roundness influencing on a rotation transmission accuracy.
For example, a conventional resin gear 100 illustrated in FIG. 12 is configured such that a web 103 couples a boss 101 to a tooth portion 102, first to third annular ribs 104 to 106 concentric with the boss 101 are formed on the web 103 with intervals from a radially inner side, and five gates 107 are opened at regular intervals in a circumferential direction of the second annular rib 105. This conventional resin gear 100 includes radial direction ribs 108 that couple the first to third annular ribs 104 to 106 in the radial direction, and the radial direction ribs 108 are formed at intermediate positions of adjacent gate marks 107a (the gates 107) and on both front and back surfaces of the web 103. On this conventional resin gear 100, a thickness of an inner web portion 110 between the first annular rib 104 and the second annular rib 105 and a thickness of an outer web portion 111 between the second annular rib 105 and the third annular rib 106 are thinnest on radial direction lines 112 passing through the gate marks 107a, and the thicknesses increase as separating from the radial direction lines 112 passing through the gate marks 107a along the circumferential direction.
A description will be given of such flow of the molten resin in an injection molding of the resin gear 100 with reference to FIG. 12A. Most of the molten resin injected into the cavity portion forming the second annular rib 105 from the five gates 107 flows along the circumferential direction of the cavity portion forming the second annular rib 105 because the thicknesses of the inner web portion 110 and the outer web portion 111 are thinner than the thickness of a portion where the second annular rib 105 is formed (the second annular rib 105 and the web 103 on which this second annular rib 105 is formed). This flow of the molten resin along the circumferential direction of the cavity portion forming the second annular rib 105 flows also in the radial direction in the flow process. However, since the thicknesses of the inner web portion 110 and the outer web portion 111 are thinnest on the radial direction lines 112 passing through the gate marks 107a, and the thicknesses of the inner web portion 110 and the outer web portion 111 increase as separating from the radial direction lines 112 passing through the gate marks 107a along the circumferential direction, the flows near the gate marks 107a in the radial direction are suppressed while the flows at the intermediate positions of the adjacent gate marks 107a and 107a in the radial direction are accelerated, thus uniformizing the amount of the flows in the radial direction. The flow of the molten resin along the circumferential direction of the cavity portion forming the second annular rib 105 joins the molten resin flowing from the other adjacent gate 107, and flows in the cavity portion forming the radial direction rib 108 in the radial direction. Thus, the molten resin flowing in the cavity portion forming the inner web portion 110, the cavity portion forming the outer web portion 111, and the cavity portion forming the radial direction rib 108 in the radial direction flows into the cavity portion forming the first annular rib 104 and the cavity portion forming the third annular rib 106, and is accumulated in the cavity portion forming the first annular rib 104 and the cavity portion forming the third annular rib 106. This equalizes the strength of the flow from the cavity portion forming the first annular rib 104 toward the cavity portion forming the boss 101 in the radial direction, and equalizes the strength of the flow from the cavity portion forming the third annular rib 106 toward the cavity portion forming the tooth portion 102 in the radial direction. Consequently, the resin gear 100 illustrated in FIG. 12 has achieved the improvement of the shape accuracy such as the roundness of the tooth portion (the outer tooth portion) 102 and the boss (the inner tooth portion) 101 (see Patent Document 1).
Patent Document 1: Japanese Patent No. 3387218